The present invention relates to the field of exhaust air systems for buildings and/or other enclosed areas, and more particularly, to exhaust discharge nozzles configured to be attached to the outlets of exhaust fans, exhaust ducts and/or stacks, and similar exhaust type equipment/devices and are specifically designed to be installed in the outdoor ambient.
Many commercial and industrial processes exist which introduce hazardous and/or noxious chemicals into the building exhaust. These chemicals originate from a host of commercial/industrial processes within critical environments such as research laboratories, chemical storage facilities, generator housing rooms, thermal oxidizers, exhaust chemical scrubbers, etc. It is of paramount importance that the proper precautions are taken to ensure that the effluent is effectively managed 100% of the time. Specifically designed, purpose built exhaust systems are required to mitigate hazardous concentrations of processes chemicals. As governed by the ASHRAE 2011 HVAC Applications Handbook, a comprehensive flow model of the building must be executed to determine critical fluid flow patterns based on the unique geometry and wind flow patterns for the site. Consideration for the location of near-by air building fresh air intakes is a critical factor which must be accounted for so as to avoid possible effluent re-entrainment into the facility in unprocessed concentrations. In order to be effective, the critical exhaust provisions must be properly designed and must achieve continuous rated performance in the real world dynamic environment where the system is to operate. Failure to meet any of the above criteria would jeopardize the safety of those working in and around the proximity of the critical environment and/or residents of surrounding communities.
An effective solution, as standardized by ASHRAE, is to propel exhaust gases upward to a critical height above the building roofline such that the effluent has exited the building boundary layer (envelope) and entered the atmospheric free stream. The atmospheric free stream essentially provides an effective mechanism to safely exhaust the effluent, by imparting sufficient plume dilution, thus reducing the concentrations of hazardous chemicals to levels deemed safe. If it is impossible or impractical to reach the atmospheric free stream, and thus plume “touchdown” is possible, then sufficient fresh air dilution must be imparted to the effluent prior to the point of discharge to ensure hazardous concentrations are sufficiently dispersed. This critical height above the building roofline is termed the “effective stack height.” In its simplest form, the effective stack height is the height at which a theoretical centerline of the building exhaust plume becomes completely horizontal due to the impact of the specified horizontal cross wind velocity. The effective stack height, hse (ft), can be calculated from the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) HVAC Applications Handbook as:hse=hs+hr−hd Where:                h is the physical exhaust system height (ft)        hr is the plume rise (ft)        hd is the amount of stack wake downwash in (ft)The plume rise component, hr, is the distance the exhaust plume will be propelled above the terminal discharge point of the physical equipment. Plume rise for momentum driven flow is calculated based on the recommendations of the ASHRAE. From as early as 1999 through 2010 the ASHRAE HVAC Handbook has stipulated the use of a special case of the Brigg's Equations to determine plume rise hr, which is defined as:hr=3.0 de(Ve/UH)Where:        de is the effective (hydraulic) diameter (ft) at the terminal discharge point of the system computed from: de=(4 Ae/π)^(½), where Ae is the cross-sectional area of the discharge opening        Ve is the equipment exit velocity (ft/min) at cross wind velocity        UH is the cross wind velocity (ft/min) at the building rooflineThis adaptation of the Briggs Equation is a function of dynamic variables. Equipment performance data must be acquired using dynamic testing parameters. Specifically, the equipment exit velocity, Ve, must be measured with the specified design cross wind, UH, applied to the system. Moreover, it is a necessary condition that the effective diameter, de, be determined for the location where the equipment exit velocity, Ve, was measured. It is recommended that this location be final discharge point (i.e. terminal location) of the exhaust system to the atmosphere. For this form of the Briggs equation for plume rise to be applicable, the discharge velocity profile at the system discharge must be characterized as uniform. A uniform velocity profile is defined as having minimal velocity gradients in the transverse plane of system discharge.        
The initial adaptation and application of the Briggs equation for plume rise did not effectively capture many critical site specific parameters, and the accepted method for calculating plume rise has been redefined in Chapter 45 of the ASHRAE 2011 HVAC Applications handbook using the Briggs equation for the vertical jet momentum of the exhaust versus downwind distance as:hr=min {βhx,βhr}                β is the stack capping factor, 1.0 without cap as in the present invention The plume rise verses downwind distance hx in (ft) is obtained from:hx=[(3Fmx)/(βj2UH2)]^(⅓)Where:        Fm is the momentum flux (ft4/s2) and is calculated as Fm=Ve2(de2/4)        βj is the jet entrainment coefficient computed as βj=⅓+(UH/Ve)        x is the downwind distanceThe final plume rise hf in (ft) is determined from:hf={0.9[Fm(UH/U*)]^(½)}/(UHβj)        
Where:                UH/U* is the he logarithmic wind profile computed as UH/U*=2.5 ln(H/z0)        H is the building height above ground level (ft)        U* is the friction velocity (ft)        z0 is the surface roughness length (ft) which can be obtained from the Atmospheric Boundary Layer Parameters Table in Chapter 45 of the ASHRAE 2011 HVAC Design Handbook.The possibility of stack wake downwash, hd, is an essential component to evaluate when computing the effective stack height of an exhaust system. Stack wake downwash occurs where arrogance low velocity exhaust streams are pulled downward by negative pressures immediately downstream of the exhaust system discharge. The amount of stack wake downwash in (ft) can be obtained from hd=de[3.0−β(Ve/UH)]        
As specified in the ASHRAE 2011 standard, the cross wind velocity at the building roofline UH, as applied to all equations which require this parameter, is the maximum design wind speed at the building roof height at which air intake contamination must be avoided. As stated by ASHRAE, this maximum design speed must be at least as large as the hourly wind speed exceeded 1% of the time. Chapter 14 of the 2009 ASHRAE Fundamentals Handbook lists this value for many cities. When determining this 1% value, the height and terrain of the building in relation to the height of the anemometer and terrain of at the local meteorological station measuring wind speed must be corrected for the effects of boundary layer friction. This is accounted for according to the following expression which appears in Chapter 24 of the 2009 ASHRAE Fundamentals handbook:UH=Umet[(δmet/Hmet)^amet]*[(H/δ)^a]Where:                Umet is the hourly wind speed as measured from a nearby meteorological station        δmet is the atmospheric boundary layer thickness of the meteorological station which is assigned based on a terrain category which can be obtained from a table of atmospheric boundary layer parameters        Hmet is the height of the anemometer (typically 33 feet)        amet is the meteorological exponent corresponding to the terrain category of the meteorological station which is assigned based on a terrain category which can be obtained from a table of atmospheric boundary layer parameters.        H is the height at which the required windspeed UH is being adjusted for; typically the discharge location of the exhaust system.        δ is the atmospheric boundary layer thickness of the exhaust system location which is assigned based on a terrain category which can be obtained from a table of atmospheric boundary layer parameters        a is the exponent corresponding to the terrain category of the location where the exhaust system is located which is assigned based on a terrain category which can be obtained from a table of atmospheric boundary layer parameters.        
Upon examination of the equation for effective stack height it becomes evident that the most critical parameters affecting a system's ability to achieve this specification are discharge geometry (de), discharge velocity (Ve), and the design wind speed (UH) where the system is to operate. Furthermore, the American National Standards Institute/American Industrial Hygiene Association ANSI/AIHA Z9.5 2012 Laboratory Ventilation standard mandates a minimum discharge velocity of 3000 ft/min be constantly maintained in order to be in compliance. Standard Z9.5 2012 also specifies that the physical exhaust system height, hs, be a minimum of 10 ft. above adjacent roof lines and air intakes and in a vertical up direction.
It should be noted that standard industry testing methods, at the present time do not incorporate cross winds into the testing protocol. The Air Movement and Control Association (AMCA) has developed AMCA Standard 260-07 Laboratory Methods of Testing Induced Flow Fans for Rating and is generally accepted as the industry standard. However, while this test does certify discharge flow volume of an induction exhaust system, it does not include dynamic testing with the influence of a cross wind. Therefore, using outlet flow data to calculate system exit velocities measured according to AMCA standard 260-07 can lead to erroneous discharge velocity ratings. Furthermore, if static system exit velocities (i.e. no cross wind present during measurement) are used in the special case Briggs Equation, which is a function of dynamic variables only, to determine plume rise, the prediction of performance will be physically incorrect. Plume rise (i.e. the quotient) determined in this manner would always be mathematically undefined (i.e infinite asymptote) due to the 0 ft/min cross wind velocity devisor; which is an impossible physical phenomena to achieve. However, if the AMCA standard 260-07 were modified to incorporate cross wind, then the Briggs equations would be a mathematically valid method of calculating plume rise, provided that the velocity profile at the discharge was uniform. Additionally, an advanced engineering approach is to use computational fluid dynamics (CFD) software to calculate system performance; the AMCA 260 test can be simulated with the cross wind component included to develop real world performance data. The Briggs equation is valid for calculating plume rise using the CFD data; however this only applies to systems with a uniform discharge profile. Additionally, the most current methodology of calculating plume rise as defined by ASHRAE should always be used.
The application of discharge nozzles at the exit point of exhaust systems enhances the performance capability by increasing discharge velocity. Increased discharge velocity provides a plume rise component of the exhaust stream with the specific goal of maximizing the exhaust/effluent dispersion and dilution of the hazardous/contaminated air and/or effluent gases and vapors from buildings, rooms, and other enclosed spaces by reaching the atmospheric free stream If it is impossible or impractical to reach the atmospheric free stream, and thus plume “touchdown” is possible, then sufficient fresh air dilution must be imparted to the effluent prior to the point of discharge to ensure hazardous concentrations are sufficiently dispersed. Discharge nozzles may be of the non-inducing or fresh air inducing type. Inducing type discharge nozzles have the unique ability of leveraging physics to draw in fresh ambient air downstream of the primary air mover (fan), while non-inducing type nozzles must draw the fresh air in through a mixing plenum and process the air through the fan, thus requiring a comparatively higher electrical power consumption. Properly designed nozzles are capable of propelling high velocity plumes of exhaust gases to heights sufficient to prevent stack wake downwash and disperse the effluent over a large upper atmospheric area, so as to avoid exhaust contaminant re-entrainment into building ventilation intake zones. Discharge nozzles are able to provide a superior alternative to conventional tall exhaust stacks which do not increase the velocity of the exhaust and thus must be significantly taller than systems discharging at a comparatively higher velocity through the use of discharge nozzles. Tall exhaust stacks are costly to construct, may be prohibited by zoning height restrictions, are visually unattractive by today's architectural standards, and may detract from community relations due to the inherent industrial connotation.
A further development of the exhaust nozzle design is the type nozzle that employs the Venturi effect to draw additional ambient air into the primary effluent stream. The venturi type nozzle can further be described as an aspirating, or induction type, as related to conventional technological description for this type nozzle. The additional induced air volume dilutes the primary exhaust gases at/near the nozzle as the combined mixed air volumes are released into the atmosphere. Also, with this exhaust-air mixture volume increase, the discharged gas is expelled at a higher velocity, achieving a greater plume height. The underlying effect of greater volume at greater discharge velocity is an increased effluent momentum, which assists with the effluent disbursement into the atmosphere.
The features and functions of induction nozzles are described in greater detail in U.S. patent application Ser. No. 13/067,269, the disclosure of which is incorporated herein by reference.
High plume lift is particularly critical with regard to exhaust gases from potentially contaminated sources, such as laboratories and other facilities in which chemical processes produce noxious fumes. To insure that potentially contaminated exhaust reaches a minimum altitude to avoid downwash, many environmental and building code standards specify a minimum discharge velocity from an exhaust nozzle. For example, ANSI Z9.5 2012 currently requires a minimum discharge velocity of 3000 feet per minute (FPM) at the outlet of a lab exhaust nozzle. Therefore, the nozzle must be designed to ensure that the discharge velocity is always at or above the governing guidelines. To achieve this, the flow rate from the fan at the inlet of the nozzle must be accelerated.
In the present invention, a frusto-conical transitional flow impinger provides a mechanism to effectively control the flow velocity in the region from the discharge of the fan impeller through the nozzle body. It is important to note, that any decrease in velocity in the region of the discharge of the fan impeller to the discharge of the nozzle body would require a subsequent re-acceleration of the air stream to initiate and sustain the fresh air inducing venturi effect at the fresh air induction ports within the nozzle. Deceleration and subsequent re-acceleration is very inefficient and increases pressure loss through the nozzle, thereby requiring more fan power, larger fan/motor sizes, and limiting usable range of the device. The addition of the impinger provides a mechanism to ensure that flow velocities are always constant or increasing until the discharge plane of the nozzle body, thereby offering a means to optimize the design of the nozzle for the given flow and/or operational pressure drop requirements.
Fans have a characteristic performance curve for a given impeller rotational speed, whereby the available static pressure is a function of flow rate. As the nozzle can be considered an accessory in the air stream, each characteristic nozzle design has an inherent initial loss at a given flow rate. The pressure drop is the consequence of the additional energy required to accelerate the airstream to the minimum acceptable discharge velocities per ASHRAE/ANSI. As more flow is introduced to the nozzle inlet, the loss (pressure drop) through the nozzle increases according to a square relationship with flow. Thus, one specific nozzle will require all of the available static pressure capacity a specific fan can provide at a given rpm well before the entire unobstructed flow range of the fan is provided. By adjusting the impinger and induction port dimensions, various nozzles that would mount to the same fan, with the same nominal exterior housing dimensions, can be efficiently designed. Thus additional flow can be introduced into the variant nozzle for the same pressure drop and corresponding minimum discharge velocity requirement. The net effect is a re-indexing of the nozzle operating range, such that additional capacity can be provided. This re-indexing strategy can be repeated as many times as necessary to completely cover a given flow range. This flow range can apply to retrofit applications or be leveraged to design nozzle sizes for a specific fan series. Moreover, fans are typically characterized in industry by their nominal impeller diameter which constitutes a given fan series. As the impeller diameter deviates from standard, which may be due to a specific performance goal or application, so will the exterior fan housing dimensions. The present invention has the unique ability to be modified so as to maintain the same engineered velocity and flow profile throughout the nozzle body and plume development chamber, effectively making true application-specific design possible